Water absorbent resin (SAP/Super Absorbent Polymer) is a water-swellable water-insoluble polymer gelling agent. The water absorbent resin is widely used, mainly disposable use, for absorbing articles such as a paper diaper and sanitary napkin, and further for an agriculture/horticulture water retaining agent, an industrial waterproofing agent, and the like. For such water absorbent resin, many monomers and hydrophilic polymers have been proposed as raw materials. Especially, polyacrylic acid (salt)-based water absorbent resin in which acrylic acid and/or its salt is used as its monomer is used most popularly in industries because of its high water absorbing ability (Non-Patent Literature 1).
The water absorbent resin is produced via a polymerizing step, a drying step, and if necessary a non-dried matter removing step, a pulverizing step, a classification step, a surface crosslinking step, or the like (Patent Literatures 1 to 5, and 50). Meanwhile, the water absorbent resin is required to have many functions (properties) in order to cope with functional sophistication of paper diapers which are one major application of the water absorbent resin. More specifically, the water absorbent resin is required to satisfy many properties such as, not only a high water absorption capacity, but also gel strength, water soluble component, a water absorbent speed, a water absorption capacity under load, liquid permeability, particle size distribution, an anti-urine property, an antimicrobial property, impact resistance (an anti-damaging property), powder fluidity, an deodorant property, anti-coloration (degree of whiteness), low dustiness, and the like. Therefore, many crosslinking techniques, additives, modifications in steps in the production, and the like have been proposed.
Among those properties, the liquid permeability is considered as a more important factor in association with a recent increase (for example, 50% by weight or more) in an amount of the water absorbent resin used in paper diapers. Furthermore, methods and techniques for improving liquid permeability against pressure and liquid permeability without load, such as SFC (Saline Flow Conductivity, see Patent Literature 6) or GBP (Gel Bed Permeability, see Patent Literatures 7 to 9), have been proposed.
Various combinations of a plurality of parameters (including the liquid permeability) of the properties have been also proposed. There have been known a technique for defining impact resistance (FI) (Patent Literature 10), a technique for defining a water absorbent speed or the like (FSR/Vortex) (Patent Literature 11), and a technique for defining the product of liquid diffusivity (SFC) and core absorption quantity after 60 minutes (DA60) (Patent Literature 12).
As the method for improving the liquid permeability such as SFC and GBP, there have been known a technique for adding plaster before or during polymerization (Patent Literature 13), a technique for adding spacers (Patent Literature 14), a technique for using a nitrogen-containing polymer having 5 to 17 [mol/kg] of nitrogen atoms which can be protonated (Patent Literature 15), a technique for using polyamine and polyvalent metal ions or polyvalent anions (Patent Literature 16), a technique for covering, with polyamine, water absorbent resin having a pH of less than 6 (Patent Literature 17), and a technique for using polyammonium carbonate (Patent Literature 18). In addition, there have been known a technique for using polyamine having water soluble component of not less than 3%, and a technique for defining a suction index (WI) or gel strength (Patent Literatures 19 to 21). There have been also known techniques for using polyvalent metal salt while controlling, during polymerization, methoxyphenol that is a polymerization inhibitor, in order to improve coloration and the liquid permeability (Patent Literatures 22 and 23). Moreover, there has been known a technique for polishing particles so as to attain a high bulk specific gravity (Patent Literature 24).
Moreover, in addition to the liquid permeability, the water absorbent speed is also a significant basic property of the water absorbent resin. As one method for improving the water absorbent speed, a technique to increase a specific surface area in order to attain a greater water absorbent speed has been known. More specifically, a technique for controlling to attain fine particle diameter (Patent Literature 25), techniques for granulating fine particles with a large surface area (Patent Literatures 26 to 28), a technique for freeze-drying a hydrogel to cause the hydrogel to be porous (Patent Literature 29), techniques for performing granulation and surface crosslinking of particles simultaneously (Patent Literatures 30 to 32), techniques for foaming polymerization (Patent Literatures 33 to 48), techniques for post-polymerization foaming and crosslinking (Patent Literature 49), and the like have been proposed.
More specifically, as to the foaming polymerization, the following techniques have been known regarding a foaming agent for treating a monomer: techniques for using a carbonate (Patent Literatures 33 to 40), techniques for using an organic solvent (Patent Literatures 41 and 42), techniques for using an inert gas (Patent Literatures 43 to 45), techniques for using an azo compound (Patent Literatures 46 and 47), and a technique for using insoluble inorganic powder (Patent Literature 48), and the like. A technique of foaming after polymerization (Patent Literature 49) and, in Patent Literature 50, a technique of controlling particle size distribution of a gel during drying (Patent Literature 50) have also been known. A technique of using a reducing agent in combination with a chelating agent, or the like for lowering residual monomers has also been known (Patent Literature 51).
Further, as an unpublished prior application on the priority date of the present application (Jun. 29, 2011), a technique of controlling internal gas bubbles ratio for improving water absorption is disclosed in Patent Literatures 52 and 53.